The physical elements in a packet-based data communication system typically include multi-port network units such as switches and hubs. Such network units are commonly and conveniently manufactured with a fixed plurality of ports by means of which the network units are connected by way of appropriate transmission media or links to other network units or users, represented by data terminals, personal computers and so on. In order to provide greater versatility in constructing the physical layout of networks by minimizing the number of different sizes of units that need to be deployed it is common to render the units ‘stackable’. This term is intended to mean that a multiplicity of such units can be connected to form effectively a single controllable entity. Thus, merely by way of example, three switches each with 24 ports may be ‘stacked’ so that effectively they constitute a single switch with 72 ports. It is customary to make such units physically stackable in a column though this physical aspect of the term ‘stackable’ is not essential to the meaning of the term or the present invention.
In order to provide intercommunication between the network units, so that for example a packet received at the port of any one of the units may be forwarded from a port or ports on a different unit or even a multiplicity of different units in the stack, depending on the addressing of the packet, it is customary to connect the stacked units together by means of a cascade, which is the term used for the communication links and associated control functions by means of which packets are transmitted from one unit to each other unit in the stack. The cascade allows users to be connected to any of the units in the stack and needs to be organised so that the stack can be managed as a single logical entity.
From the operational point of view, the forwarding of packets on the cascade has to be controlled according to a variety of rules which are intended to conform to the forwarding rules relevant to different types of packets (such as unicast, multicast and broadcast) as well as other relevant rules, such as those relating to bridging. An example of such a rule is the prevention of the forwarding of a packet out on a port by which it has been received. Such rules may need to be modified to make the operation of the cascade more convenient or more versatile, as exemplified in prior copending patent applications Ser. No. 09/511,118 filed 23 Feb. 2000 by Drummond Murray et al and Ser. No. 09/668,339 filed 25 Sep. 2000 by O'Keeffe et al (now U.S. Pat. No. 6,801,950 issued Oct. 5, 2004).
Two variable values which are the basis of the operational control of a stack are the unit numbering, namely some numerical or coded quantity which identifies and distinguishes each physical unit in the stack and an ‘active unit’ total, which indicates the number of units actively participating in the stack. It is desirable to compute the unit numbering of the units in a stack and the active unit total and to convey the active unit total to all units in the stack automatically.
It is possible to employ an ‘in-band’ configuration in which the configuration process by means of which the units are allotted their stack identification by means of configuration packets sent during a configuration routine on the data path for packets connecting the unit in the stack. Such a system is described, for example, in GB published Patent Application No. 2338155, which describes a cascade connection in which packets are employed to configure the cascade such that only one unit, called the master, is allowed to place packets on the cascade connection at any time. The prior copending Patent Application for Byham et al, Ser. No. 09/369,323 filed 6 Aug. 1999 describes an automatic configuration process by means of which configuration packets are used to determine which unit is deemed to be at the bottom of the stack not withstanding the provision of resilience or possible misconnection of connecting cables.
Prior co-pending patent application Ser. No. 09/662,158 for Poulter et al, filed 1 Aug. 2000, entitled ‘Cascade Architecture and Operation for Packet-based Communication Systems’ and fully incorporated by reference herein describes a system, devised by various inventors common to the inventors of the present Invention, in which control messages travel around the cascade connection by a control path distinct from the path of data packets and are employed in the establishment of the identification numbers for the units in the stack and are also employed for computing an ‘active unit total’. The system described in that application is a particularly versatile system which can accommodate the ‘hot’ insertion or removal of active units and which can be employed to compute the ‘active unit total’ which takes into account the units which are within the cascade but which may be powered down or otherwise not actively participating in the cascade.
The physical architecture required in the earlier application Ser. No. 09/662,158 is, as far as its preferred implementation is concerned, based on the use of special three-port connectors which define paths for the packets and the control messages and is furthermore organised so that if a unit which is connected to a connector is powered down or inoperative, the system still provides, by means of the relevant connector, both an unbroken data path and all unbroken control path around the cascade. That earlier application also describes ‘cascade modules’ by which a network unit can be connected in the cascade.
There is however a need for a system which provides at least some of the functionality of the earlier system, and in particular the computation of unit numbering and an active unit total, and which can be implemented in circumstances where the earlier system is either not feasible or not needed. For example if the available terminal connectors have insufficient terminals for supporting the variety of signals and control paths required in practice for the system described in the co-pending application, the system described in Ser. No. 09/662,158 cannot be satisfactorily employed. Furthermore, the ‘hot swap’ facility might not be needed. As will become more apparent, the present invention may be implemented in a simple manner without a ‘hot swap’ facility, though that facility may be provided within the scope of the invention.